Recently, as an electrochemical element, a lithium ion secondary battery represented by non-aqueous electrolyte secondary battery is widely used because it is light in weight, high in electromotive force, and high in energy density. For example, the demand for lithium secondary battery is increasing for use as driving power source for portable telephone, digital camera, video camera, laptop computer, and other various portable electronic devices and mobile communication appliances.
The lithium ion secondary battery is composed of a positive electrode made of composite oxide containing lithium, a negative electrode included lithium metal, lithium alloy or a negative electrode active material capable of inserting and extracting lithium ions, and an electrolyte.
Recently, instead of the conventional negative electrode material such as graphite and carbon material, new elements having a lithium ion inserting property and having a theoretical capacity density exceeding 833 mAh/cm3 are reported. Elements of negative electrode active material and having a theoretical capacity density exceeding 833 mAh/cm3 include silicon (Si), tin (Sn), and germanium (Ge) alloying with lithium, and their oxides and alloys. In particular, since particles containing silicon such as Si particles or silicon oxide particles are inexpensive, they have been widely studied.
These elements are, however, increased in the volume when inserting lithium ions in the charging process. For example, when Si is used as negative electrode active material, when lithium ions are inserted to a maximum extent, the compound is expressed as Li4.4Si, and in the process of transforming from Si to Li4.4Si, the volume is increased 4.12 times of discharged state.
In particular, when a negative electrode active material is formed by depositing a thin film of the element on the current collector by a CVD method or a sputtering method or the like, the negative electrode active material is expanded and contracted by inserting and extracting lithium ions. In the repeating period of charging and discharging cycles, the adhesion of the negative electrode active material and the negative electrode current collector is lowered, and peeling may occur.
To solve the problems, various methods have been proposed, such as a method of forming undulations on the surface of the current collector, depositing a thin film of negative electrode active material, and forming pores in the thickness direction by etching (see, for example, patent document 1). Other example is a method of forming undulations on the surface of the current collector, forming a resist pattern so that the convex portions may correspond to the opening, forming a thin film of negative electrode active material thereon electrolytically, and removing the resist to form columnar bodies (see, for example, patent document 2).
Another example is a method of forming a film of active material containing silicon and oxygen by varying the oxygen ratio on the surface of the current collector, and forming the film of active material layer higher in the oxygen ratio and smaller in expansion and contraction by insertion and extraction of lithium ions near the current collector (see, for example, patent document 3). As a result, expansion and contraction of active material layer at the interface of the current collector can be suppressed, and occurrence of creasing and pealing may be suppressed.
That is, in the secondary batteries disclosed in patent document 1 and patent document 2, a thin film of negative electrode active material is formed in columnar bodies, pores are formed among columnar bodies to prevent peeling or creasing. However, since the composition of columnar negative electrode active material is uniform, by insertion and extraction of lithium ions, the columnar thin film near the interface of the current collector is similarly expanded and contracted. As a result, as compared with the case of forming on the entire surface, the effect is suppressed, but a stress similarly occurs at the interface of columnar thin film and current collector, and peeling may occur, and substantial improvement of cycle characteristics is not expected. Further, in order to increase the battery capacity, if the height of the columnar thin film is increased and intervals of pores are narrowed, in particular, the leading end (open side) of the columnar thin film is not regulated by the current collector, and along with progress of charge, the columnar thin film is expanded largely as compared with the vicinity of the current collector. As a result, the columnar thin films mutually contact and push near the leading end, and peeling of columnar thin film from the current collector or creasing of current collector may occur. Therefore, both prevention of peeling of columnar thin film from the current collector or creasing of current collector and enhancement of battery capacity cannot be realized at the same time.
In the secondary battery of patent document 3, the active material layer is made of silicon and oxygen, and the oxygen ratio of active material layer near the interface of the current collector is heightened. As a result, occurrence of stress due to expansion and contraction of active material layer at the interface is suppressed. However, the active material layer apart from the interface of the current collector is expanded in its volume by insertion of lithium ions. By expansion of volume, the current collector may be creased or deformed. The leading end of the active material layer may be expanded, but the active material layer near the interface of the current collector is hardly expanded, and along with the progress of charging and discharging cycles, the active material layer may be torn or peeled. In particular, when the active material layer is increased in thickness, this effect becomes more obvious, and there is a problem for enhancement of capacity. In patent document 3, it is also proposed to form undulations on the surface of the current collector, and form an active material layer having undulations along the undulations, but essential the same problems exist.    Patent document 1: Unexamined Japanese Patent Publication No. 2003-17040    Patent document 2: Unexamined Japanese Patent Publication No. 2004-127561    Patent document 3: Unexamined Japanese Patent Publication No. 2006-164954